1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an inkjet recording method using an inkjet recording head that discharges or ejects ink droplets for recording or printing an image.
2. Description of the Related Art
The inkjet recording apparatus is advantageous because it is low in noise and running costs. Also, it is easy to downsize an apparatus body and can be used for color printing. Inkjet recording apparatuses are widely used for printers, copying machines, and facsimiles. In present inkjet recording apparatuses, ink droplets discharged from a recording head are smaller and therefore form a small dot that can reduce the granularity on a recorded or printed image. Especially, the present recording heads tend to discharge small droplets of color inks. In recent years, the droplet size has decreased from 15 pl to 5 pl or to 2 pl.
Meanwhile, highly advanced digital input devices are widely used for high-quality images. The inkjet recording apparatuses are thus required to output high-quality images comparable with the images entered from the digital input devices. From these circumstances, in consideration of the matching with the advanced input devices, the inkjet recording apparatuses must be sufficiently reliable for realizing small droplets and the nozzle opening diameter of a recording head must be reduced in accordance with the droplet size.
However, as the ink droplet becomes smaller, the kinetic energy of a droplet becomes smaller. The droplet is easily affected by the surroundings. For example, when an ink droplet is discharged at a high density, i.e., when a recording operation is performed at a higher recording duty, an air stream generated around a recording head possibly bends a flying direction of an ink droplet. More specifically, the flying direction of an ink droplet discharged from an edge portion of a nozzle array of the recording head is offset or deviated toward the center of the nozzle array under the influence of air stream. Accordingly, the ink impact position deviates from an intended position on a recording medium. The phenomenon appears significantly at an edge portion of a nozzle array of the recording head.
To solve such an impact deviation, Japanese Patent Application Laid-open No. 2003-145775 (corresponding to US Patent Application No. 2003-067508) discloses a wide clearance between an endmost nozzle (i.e., a nozzle positioned at an endmost portion of a nozzle array provided on a recording head) and a neighboring nozzle. Providing a wide clearance between the endmost and neighboring nozzles enables for correction of the impact position of an ink droplet which is forcibly shifted toward the center of the nozzle array due to an air stream generated during a high recording duty operation.
However, the correcting method proposed in Japanese Patent Application Laid-open No. 2003-145775 is based on an assumption that the impact position of an ink droplet discharged from the edge portion of a nozzle array is greatly offset or deviated during a high recording duty operation. Therefore, this method is practically used only for a high recording duty operation. If the recording duty is low, this method will cause an unexpected counter-deviation of an ink droplet discharged from the edge portion of a nozzle array. More specifically, when an image is formed in a low recording duty, an air stream is hardly generated. An ink droplet discharged from the edge portion of a nozzle array is not so affected by the air stream.
Hence, the flying direction of an ink droplet does not bend as much. As a result, the impact position of an ink droplet deviates from an intended position by an amount equivalent to a widened pitch between the endmost and neighboring nozzles. Such a counter-deviation of the impact position appears as unevenness of concentration on an image. Especially, in speedy recording operations in a single- or 2-pass mode, the unevenness of concentration appears as a black streak along the boundary of lines. As a result, the image quality deteriorates.
The unevenness of concentration will be described in more detail. It is now assumed that a recording head is scanned twice to record the data of a head width. For the above scanning operation, a recording head is used in which an amount of an ink droplet is 1.5 pl, a nozzle alignment density is 1200 dpi, a total nozzle number is 256, and a width in a nozzle alignment direction is 0.21 inches. In this case, in order to form an image on a recordable region (8×11 inches) of an A4-size recording medium, the scanning of the recording head must be repeated approximately 104 times. The recording head is driven at a drive frequency of 30 kH (that is, a moving speed of the recording head is 25 inch/sec).
When the recording duty is high, i.e. approximately 50% or above, the flying direction of an ink droplet discharged from the edge portion of a nozzle array of the recording head undesirably bends toward the center of the nozzle array due to the effect of an air stream. As a result, the impact position of the ink droplet discharged from the edge portion of the nozzle array is offset or deviated toward the center of the nozzle array. However, the nozzle pitch at the edge portion of a nozzle array is widened beforehand as described above. Thus, a deviation amount of the ink droplet is canceled by the wide nozzle pitch. As a result, the impact position of an ink droplet becomes correct and no streak appears on the boundary between each line.
On the other hand, when the recording duty is reduced to approximately 50% or less, an air stream is hardly generated around the recording head even if the recording is performed using the same recording head under the same recording conditions. Accordingly, the deviation amount of an ink droplet discharged from the edge portion of a nozzle array is small. In this case, due to the wide nozzle pitch at the edge portion of a nozzle array, the dots formed by the ink droplets discharged from the end nozzles are overlapped with each other. Accordingly, a black streak appears on the boundary between each line.